On Thu, Jul 20, 2017 at 01:24:49PM +0200, Peter Zijlstra wrote:
> ~/tmp$ gcc -o sqrt sqrt.c -lm -O2 -DLOOPS=10000000 -DNEW=1 -DFLS=1 -DANSHUL=1
> ; perf stat --repeat 10 -e cycles:u -e instructions:u ./sqrt
>
> Performance counter stats for './sqrt' (10 runs):
>
> 328,415,775 cycles:u
> ( +- 0.15% )
> 1,138,579,704 instructions:u # 3.47 insn per cycle
> ( +- 0.00% )
>
> 0.088703205 seconds time elapsed
> static __always_inline unsigned long fls(unsigned long word)
> {
> asm("rep; bsr %1,%0"
> : "=r" (word)
> : "rm" (word));
> return BITS_PER_LONG - 1 - word;
> }
That is actually "lzcnt", if I used the regular fls implementation:
static __always_inline unsigned long __fls(unsigned long word)
{
asm("bsr %1,%0"
: "=r" (word)
: "rm" (word));
return word;
}
It ends up slightly more expensive:
~/tmp$ gcc -o sqrt sqrt.c -lm -O2 -DLOOPS=10000000 -DNEW=1 -DFLS=1 -DANSHUL=1
; perf stat --repeat 10 -e cycles:u -e instructions:u ./sqrt
Performance counter stats for './sqrt' (10 runs):
384,842,215 cycles:u
( +- 0.08% )
1,118,579,712 instructions:u # 2.91 insn per cycle
( +- 0.00% )
0.103018001 seconds time elapsed
Still loads cheaper than pretty much any other combination.
